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2024-09-02
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1 设备概况
6 kV交流异步电机，额定功率为725 kW、额定转速1490 r/min，冷却方式为IC411。
2 故障现象
该电机大修后，返厂后运行1个月左右，运维人员反映该电机在运行时发出异常噪声，且呈逐渐增大趋势，并伴随着明显的啸叫声。
经过初步检查，电机各位置振动速度均不超过1.5 mm/s，电机轴承润滑良好。用简易测振仪器检测电机振动速度时，发现最高振动值出现在500 Hz附近（下图），且远大于其它频率。这个频率远大于电机转速的1倍频和2倍频，而1倍频和2倍频的振动主要与电机轴承损坏、动不平衡、不对中、地脚缺陷相关。由此判断，该噪声应该与机械振动无关。
电机电磁噪声大小随磁场强弱、负载电流的大小而改变。做停电试验，电机停运的瞬间，异常噪声并未立即消失，而是伴随电机转速的下降而减弱，可以据此确定该异常噪声不是电磁噪声。
3 解体检查
电机停运后，运维人员打开风扇罩，对风扇叶和风扇罩内部进行检查，未发现风扇叶松动、开裂的现象，风扇罩内部无任何异物，可以判断噪声来源于电机内部。
拆开电机两侧端盖，抽出电机转子，电机内部轴承、绕组均未见异常，仅在非轴伸端转子内风扇底部发现有绝缘漆滴落后开裂现象（下图）。该绝缘漆是电机大修时真空浸漆后，绝缘漆由于重力作用滴落在定子壳体上，修理厂检修人员未及时清理干净所致。
4 故障原因分析
经过测量，电机各项电气指标均正常。通过对电机转子的结构进行了观察与分析，发现在转子上有一圈管道连接了转子的两端，转子转动时起通风散热作用（下图）。
每当转子旋转一周，会有18个小孔经过开裂的绝缘漆层位置，小孔内的气流吹过形成了“口哨效应”。经过简单计算:每秒电机转动1490÷60=24.83圈；每秒会有18x24.83=447个小孔吹过绝缘漆层开裂位置，可以理解为气流吹过该位置的频率为447 Hz。
空气动力噪声分为涡流噪声和笛鸣噪声两种。涡流噪声主要是由转子和风扇引起的冷却空气湍流，在旋转表面交替出现涡流引起的；笛鸣噪声是通过压缩空气或空气在固定障碍物上擦过而产生的。
这个447 Hz频率数字接近主振频率（500 Hz），电机噪声的来源基本确认可能与开裂的绝缘漆层有关。如果把开裂的绝缘漆层清理干净，噪声应会消失。
5 效果及改进措施
清除了滴落在定子壳体内部的绝缘漆层，将电机回装后试车，发现异常噪声消失，测量振动的频谱，500 Hz处的没有任何反应（图5），可以确认该电机之前发出的噪声确实为空气动力性噪声，即空气在开裂的绝缘漆皮上擦过而产生的笛鸣噪声。
个人对该文章的分析有一点看法：首先500HZ是由于流体通过开裂的绝缘漆造成的，也就是流体通过障碍物后产生的噪声。根据散热孔计算的447 Hz，反而说明散热孔的设计与、运行是正常的。
文中所述的机械原因排除，理由并不充分，当某部件共振时，可能会产生高的振动频率。
另外怀疑是否为空气动力噪声，最简单的方法是在有条件的情况下停掉风扇观察。
来源：振动诊断与转子平衡
（版权归原作者或机构所有）
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